The principles of photothermal spectroscopy are generally described in a publication by Stephen E. Bialkowski entitled “Photothermal Spectroscopy Methods for Chemical Analysis”, John Wiley & Sons, Inc., 1996, the entire content of which is incorporated by reference herein. Photothermal spectroscopy method provides sensitive measurements of optical absorption in homogeneous and inhomogenous media.
McLean et al. (E. A. McLean et al. American Journal Applied Physics Letters, 13, p. 369 (1968)) recognized that the optical absorption resulting in sample heating and subsequent changes in refractive index would cause a phase shift in light passing through the heated region. This phase shift can be detected by interferometric means.
Grabiner et al. (F. R. Grabiner et al. Chemical Physics Letters, 17, p. 189 (1972)) proposed to use two lasers for photothermal interferometric spectroscopy: pulsed infrared laser for the medium excitation and visible probe laser for the refractive index change measurement.
In the U.S. Pat. No. 6,709,857 a system and method for monitoring the concentration of a medium using photothermal spectroscopy is disclosed. The system and method each employs an energy emitting device, such as a laser or any other suitable type of light emitting device, which is adapted to emit a first energy signal toward a location in the container. The first energy signal has a wavelength that is substantially equal to a wavelength at which the medium absorbs the first energy signal so that absorption of the first energy signal changes a refractive index of a portion of the medium. The system and method each also employs a second energy emitting device, adapted to emit a second energy signal toward the portion of the medium while the refractive index of the portion is changed by the first energy signal, and a detector, adapted to detect a portion of the second energy signal that passes through the portion of the medium. The system and method each further employs a signal analyzer, adapted to analyze the detected portion of the second energy signal to determine an amount of a sample in the container based on a concentration of the medium in the container.
In standoff chemical sensing method using photothermal interferometric detection has been disclosed by the same inventive entity as the present invention in U.S. Pat. No. 7,426,035, which is fully incorporated herein by reference. The system includes a strobe unit and a probe unit. The strobe beam changes the refractive index of chemical under study, and the probe beam reads out the information about the refractive index change. The probe subsystem employs a phase-diversity scheme where a six-port optical 90° hybrid is used to combine the interrogation probe beam and the reference or local oscillator (LO) laser beams. Two sets of balanced photoreceivers are employed to obtain two quadrature-phase homodyne signals, namely, I=A cos(φn+φs) and Q=A sin(φn+φs) where A is a proportion constant which depends on the square root of the received probe laser power.
There is a need for reliable remote methods and systems for detecting the presence of chemicals in the field. There is a need to provide highly sensitive receivers to improve signal-to-noise ratio of the detected signal, which gives an opportunity to detect traces of chemicals remotely.